Ever since the 1940's when retroreflective sheeting based on a monolayer of embedded microsphere-lens elements was introduced and came into widespread use, there has been a desire for glass microspheres having increased indexes of refraction. Referring to the illustration of embedded microsphere sheeting in FIG. 1 of the attached drawing, the higher the index of refraction of the microspheres (M) with respect to the index of refraction of the transparent flat-surfaced top layer (T), the thinner the transparent spacing layer (S) can be; and if the ratio of microsphere and top layer indices of refraction were high enough (slightly over 1.9), the spacing layer could be eliminated. Most transparent polymeric materials such as used for the top layer have an index of refraction around 1.5, which would call for microspheres having an index of refraction of about 2.9 in order to eliminate the spacing layer.
Despite the desire for high-index microspheres, the art has never, insofar as known, provided commercially useful glass microspheres having a refractive index higher than about 2.7. The highest-index prior-art microspheres have been based on large amounts of Bi.sub.2 O.sub.3 or PbO, usually in combination with rather large amounts of TiO.sub.2 ; see Beck et al, U.S. Pat. No. 2,726,161; Beck et al, U.S. Pat. No. 2,853,393, which teaches some PbO-Bi.sub.2 O.sub.3 -based examples having indices up to 2.67; and Stradley et al, U.S. Pat. No. 2,870,030, which teaches some Bi.sub.2 O.sub.3 -TiO.sub.2 -based examples having indices up to 2.59. None of these prior-art microspheres have been commercialized, for one reason, because the index of refraction obtained is not high enough to eliminate or sufficiently reduce the thickness of the spacing layer. Also, the PbO-containing products are now considered especially unsuited to commercial use for pollution reasons.
Searight et al, U.S. Pat. No. 3,560,074, states that glass microspheres made from 95-100 weight-percent TiO.sub.2 would have a refractive index of 2.9, but insofar as known, glass microspheres of such composition or index have never been made available to the retroreflective sheeting art. British Pat. No. 1,472,431 teaches microspheres that comprise over 50 weight-percent polycrystalline titanium dioxide in anatase form, but no commercial microspheres of this type have been supplied.
A different prior-art approach to increased refractive index, taught in Tung et al, U.S. Pat. No. 3,149,016, is a process for heat-treating glass microspheres to cause a structural rearrangement within the microspheres. The structural rearrangement, which is suggested in the patent to be development of crystal nuclei, increases the index of refraction of the microspheres. The highest-index microspheres exemplified in the patent, having an index raised from 2.47 to 2.7 by the heat-treatment process, comprise 67.5 parts PbO, and, as noted above, this dependence on PbO limits the utility of the microspheres.
The heat-treatment process is used to prepare the microspheres that are used most extensively in commercial reflective sheeting of the type shown in FIG. 1. These microspheres, taught in Tung et al, U.S. Pat. No. 3,493,403, have an index of refraction of about 2.3, and a spacing layer is included in the sheeting.
An alternative search for high index is represented by teachings of composite spheres that use a lower-refractive-index shell on a higher-index core to achieve an effective index of 2.9; see Taylor, U.S. Pat. No. 2,713,286 (an outer glass layer is leached to make it porous and of lower-refractive index) and McKenzie, U.S. Pat. No. 3,025,764 (silica-based low-index material coated on higher-index core). However, composite microspheres have not come into use, at least partially because they are costly to make.
A different approach to eliminating a space coat is described in Bingham et al, U.S. Pat. No. 3,551,025, which teaches sheeting as shown in FIG. 1 having a highly fluorinated elastomeric top coat. The index of refraction of the fluorinated compositions is sufficiently low as to form a desired ratio with the refractive index of lower index glass microspheres. However, sheeting as described in the patent is costly and it has never come into widespread commercial use.
In sum, the art has never achieved on embedded-microsphere retroreflective sheeting of widespread commercial acceptability that eliminates or greatly reduces the thickness of the spacing layer. There would be many advantages to such an elimination or reduced thickness-economies in processing time and in the amount of materials needed; improvement in properties of the sheeting such as flexibility; avoidance of variations in reflective brightness that occur with the variations in thickness of the space coat that inevitably creep into a factory operation; and longer useful life of the sheeting by eliminating the effect of weathering on the space coat. Despite the strong motivation such advantages provide, no one has ever achieved them, and a major reason is the absence of commercially acceptable glass microspheres of high enough index of refraction.